This invention relates generally to the art of winding a web of sheetlike material and more particularly to a new winding core for use in handling such material.
In the art of producing thermoplastic film, many complex and involved measures have been taken in order to wind such film about a core to produce a roll with a substantially uniform profile. One of these techniques is disclosed in U.S. Pat. No. 2,844,846. The technique disclosed therein comprises extrusion of the thermoplastic film into a bubble prior to passing the tubular extrudate through nip rolls. The extrusion die is rotated relative to the nip rolls such that any imperfection which might be created from the die configuration will be evenly distributed about a roll resulting from the film. The even distribution of slight imperfections is an extremely important aspect of winding thermoplastic film. When such imperfections are allowed to concentrate and pile upon one another, the magnitude of a slight imperfection becomes greatly exaggerated and magnified after several thousand windings. By evenly distributing such imperfections transversely across a roll such magnification is avoided. Another technique for producing a roll of thermoplastic film material having a uniformly distributed imperfections is disclosed in U.S. Pat. No. 3,161,942. The process disclosed therein comprises rotation of the winding roll rather than rotation of the extrusion die.
Another technique for improving the profile of the rolled sheet of film is disclosed in U.S. Pat. No. 3,052,917. The process disclosed therein comprises pulling the edge portion of film from the die at a more rapid rate than the central portion. The net effect of this process is the pulling of wrinkles and distortions from the film prior to rolling.
While all of these techniques confront the problems associated with the placement and winding of a sheet of thermoplastic film about a core or mandrel, certain phenomina occur once a heat shrinkable film is wound about a core. Heretofore the prior art has not confronted the problems associated with film distortions which occur after the winding of the film onto a core for storage in handling.